This invention relates to a direct image printing plate and more, particularly, it relates to a direct image printing plate from which a printing plate can be directly made by impact type printers such as type printer, wire dot printer, etc.
At present as offset printing plates used in the field of light printing there are those of (1) direct image type comprising a water-resistant support and an image receiving layer provided thereon, (2) electrophotographic type comprising a water-resistant support and a photoconductive layer provided thereon, (3) silver salt photographic type comprising a water-resistant support and a silver halide emulsion layer provided thereon, etc. Of these types of the master plates those of the type (1) have some advantages over those of (2) and (3) in that printing plates can be directly made by handwriting on the image receiving layer with oily inks or by PPC machine.
Due to the recent spread of office automation devices, especially word processors, there have been demanded direct image printing plates from which printing plates can be directly made by these devices. In case of plate-making by word processors, impact type printers such as type printers, wire dot printers, etc. are often used and direct imaging is made on the direct image printing plates through a fabric ink ribbon impregnated with a printer ink comprising coloring matters such as pigments, oil-soluble dyes, etc. and solvents such as mineral oils, vegetable oils, etc. The printer ink transferred to the image receiving layer by the impression is fixed thereon and exhibits oleophilicity there and forms image areas for printing. It is important for obtaining clear prints that the image receiving layer of the direct image printing plates have good fixing property for thus transferred printer ink.
The conventional direct image printing plates comprise a water-resistant support having thereon an image receiving layer mainly composed of an inorganic pigment such as kaolin clay, zinc oxide or the like and a binder comprising a water-soluble high polymer compound such as polyvinyl alcohol, starch, carboxymethyl cellulose or the like. This image receiving layer is generally required to have hydrophilicity and simultaneously water-resistance and besides fixability for printer ink, etc. However, the hydrophilicity and the water-resistance antipodal to each other and there have been the problems that increase in the water-resistance leads to insufficiency in hydrophilicity to often cause scumming. On the contrary, increase in hydrophilicity leads to insufficiency in water-resistance to often cause peeling of the image receiving layer. Therefore, in order to obtain image receiving layers having both the hydrophilicity and the water-resistance, there have been proposed use of two water-soluble high polymer compounds in admixture as a binder such as a mixture of carboxymethyl cellulose and polyamide epichlorohydrin, for example, in Japanese Patent Unexamined Pulbication (Kokai) No. 1791/82 and a mixture of carboxymethyl cellulose and polyglycidyl ether compound, for example, in Japanese Patent Unexamined Publication (Kokai) No. 135190/84.
However, image receiving layers containing these mixtures as binders show good fixability for toners in PPC copiers, but very low fixability for printer ink and printing plates having these image receiving layers cannot provide clear prints. That is, the conventional direct image printing plates having both the hydrophilicity and the water-resistance have the defect that they do not have good fixability for printer ink.